How Does Pulse Oximetry Work? We'll Explain the Functionality

Wondering how does pulse oximetry work? Pulse oximetry is a diagnostic procedure that is used for checking the percentage of hemoglobin that is saturated with oxygen. Continue reading to find out more about pulse oximetry.

HealthHearty Staff

Last Updated: Apr 22, 2018

Pulse oximetry is a diagnostic procedure that is used for checking the arterial oxygen saturation of hemoglobin. A pulse oximeter is a medical device that is often used in medical settings, especially for monitoring the pulse rate and amount of oxygen carried by the hemoglobin. How does pulse oximetry work and what do pulse oximetry readings signify? Scroll down so as to understand how does a pulse oximeter work along with the significance of this diagnostic procedure.

Why is Pulse Oximetry Recommended?

Oxygen is an element that is vital for one's survival. As we breathe in air, the air travels into the lungs through bronchi, and the vital respiratory process of exchange of oxygen and carbon dioxide takes place within the microscopic alveolar sacs located in lungs. Oxygen from the inhaled air gets bound to hemoglobin. The heart, which is the most vital organ of the circulatory system, carries the oxygenated blood throughout the body. If the respiratory organs are unable to work, and the oxygen-replenished blood is unable to reach the various parts of the body, then a variety of vital bodily processes would surely get affected. If the levels of life-giving oxygen fall down to dangerously low levels, one may suffer from hypoxemia. Anxiety, disorientation, bluish discoloration of skin, palpitations or respiratory distress are some of the symptoms that may be caused due to low levels of oxygen in arterial blood. While blood tests can provide an insight into the hemoglobin levels, doctors may also conduct a pulse oximetry in order to ascertain the arterial oxygen saturation levels.

What do Pulse Oximetry Readings Signify?

Pulse oximetry is a non-invasive procedure that can help the doctors in determining the amount of oxygen that is actually carried by the hemoglobin in the red blood cells. Oxygen saturation values are determined by checking the ratio of quantity of hemoglobin that is carrying oxygen and the total amount of hemoglobin in one's blood. Under normal circumstances, pulse oximetry values must lie between 95-99%. If the pulse oximetry results indicate blood oxygen saturation levels below 90%, a life-threatening situation can arise. In such cases, patient would be given supplemental oxygen so as to correct the imbalance in the blood oxygen levels. Oxygen saturation levels can be adversely affected due to ailments associated with the lungs or the heart. High altitudes, injury or carbon monoxide poisoning could also be responsible for low blood oxygen saturation. Under such circumstances, pulse oximetry levels would be lower than the normal range.

How Does a Pulse Oximeter Work

Now that you know what is meant by blood oxygen saturation, let's find out how blood oxygen saturation levels are measured by a pulse oximeter. For this medical device to function properly, it must be clipped onto a part of the body with a proper and rhythmic pulsation. A pulse oximeter consists of a peripheral probe along with a microprocessor unit. The probe could be clipped onto one's toe, finger or earlobe. If a finger pulse oximeter is being used, then a sensor probe is clipped onto one's index finger, over the fingernail. A pulse oximeter consists of a pair of light-emitting diodes. One of the diodes transmits a beam of light with red wavelengths, while the other transmits light with infrared wavelengths. Both of these diodes face a photodiode that receives the wavelengths of light emitted by the two diodes and measures and converts the values before they flash on the LCD of the device. The absorption of these wavelengths is different in case of oxygen-depleted and oxygen-rich blood. While the oxygenated blood absorbs the infrared light, it allows red light to pass through. On the other hand, oxygen-depleted blood absorbs the red light and allows the infrared light to pass through. The ratio of absorption of the red light and the infrared light can help in calculating the amount of oxygen that is carried by hemoglobin.

Limitations of Pulse Oximetry

Though a pulse oximeter is definitely a useful diagnostic tool, at times, it may not give accurate readings. First of all, it is extremely essential that this device is clipped to a part of the body with a proper pulse. If a person is suffering from peripheral vasoconstriction and doesn't have a pulsatile flow, then the pulse oximeter readings are most likely to get affected and may not be correct. It's also essential that the lights in the surroundings don't shine on the sensor and affect its ability to measure the light transmitted from the arterial blood. The readings may also get affected, if the patient is shaking rapidly or having a seizure. If the patient is wearing a dark nail polish, the pulse oximeter may not be able to provide accurate readings. If the concentration of carbon monoxide in the air is high, then the readings may again be skewed. Inhalation of smoke from vehicles, fires or cigarettes can elevate the levels of carboxy-hemoglobin, which in turn, may affect the pulse oximetry readings. Under these circumstances, an arterial blood gas test can be conducted. This is another diagnostic procedure that may be used for determining the oxygen content as well as oxygen saturation values. Unlike most of the blood tests wherein the blood sample is taken from a vein, in an arterial blood gas test, the blood has to be drawn from an artery.

Though a pulse oximeter is often used in medical settings, especially in case of patients suffering from respiratory distress, it's extremely important that this device is used properly and the shortcomings of pulse oximetry are also taken into consideration. Bright overhead lights, poor blood circulation or any other factor that may interfere with the working of this device must be attended to, before the readings are taken.